Complementary Metal-Oxide Semiconductor (CMOS)
The configuration data for a PC is stored by the BIOS in what is calledCMOS(pronounced
sea-moss, meaningComplementary Metal-Oxide Semiconductor). CMOS is also known as
NVRAM (nonvolatile RAM). CMOS is a type of memory that requires very little power
(about one-millionth of an amp) to retain any data stored on it. CMOS can store a PC’s
configuration data for many years with power from low voltage dry cell or lithium batteries.
Actually, CMOS is the technology that is used to manufacture the transistors used in
memory and IC chips. However, the name CMOS, because it was used early on for storing
the system configuration, has become synonymous with the BIOS configuration data.
The BIOS CMOS memory stores the system configuration, including any modifica-
tions made to the system, its hard drives, peripheral settings, or other settings. The
system and RTC (real-time clock) settings are also stored in the CMOS.
The information on the computer’s hardware is stored in the computer’s CMOS
memory. Originally, CMOS technology was used only for storing the system setu pinfor-
mation. Although most circuits on the computer are now made using this technology, the
name CMOS usually refers to the storage of the computer’s hardware configuration data.
When the computer is started up, the CMOS data is read and used as a checklist to verify
that the devices indicated are in fact present and operating. Once the hardware check is
completed, the BIOS loads the operating system and passes control of the computer to it.
Fromthatpointon,theBIOSisavailabletoacceptrequestsfromdevicedriversandappli-
cation programs for hardware assistance.ROM BIOS
Because the BIOS programs must be available to the processor each time it starts up, the
BIOS is stored on a ROM chi plocated on the motherboard. From this ROM chi p, the BIOS
program is loaded into a reserved area of system memory, normally the last 64KB (mem-
ory addresses F000h to FFFFh) of the first 1MB in system memory. Microprocessor and
BIOSproducershavesetthislocationinmemoryasadefactostandard,whichmeansthat(^120) PC Hardware: A Beginner’s Guide
Figure 6-5. An EPROM chip showing its erasing window